1. Field of the Invention
The present invention relates to a heat fixing device and an image forming apparatus provided with the heat fixing device.
2. Description of Related Art
An image forming apparatus such as a laser printer is usually provided with a heat fixing device having a heating roller and a press roller for heating and fixing a toner image that has been transferred onto paper. In the heat fixing device, the toner image transferred onto the paper is heated and fixed as the paper passes between the heating roller and the press roller.
The heating roller of such a heat fixing device usually has a heater such as a halogen lamp incorporated therein and a temperature sensor provided for sensing the temperature of the surface of the heating roller. In the heating roller, the turning on and off of the heater is controlled by sensing the surface temperature by a temperature sensor, to maintain a predetermined heat fixing temperature.
Two known types of temperature sensor are used in a heat fixing device: a contact type of temperature sensor such as a thermistor that is in contact with the heating roller, and a non-contact type of temperature sensor such as an infrared sensor that is not in contact with the heating roller. The contact type of temperature sensor, however, may raise problems in that the sensing accuracy thereof may deteriorate if toner adheres to the temperature sensor, and that the toner adhering to the temperature sensor may peel off to damage the heating roller or fall onto the paper to dirty it. In contrast thereto, since a non-contact type of temperature sensor does not come into contact with the surface of the heating roller, it is unlikely to damage the surface of the heating roller over time. Thus, there have been various proposals for a heat fixing device provided with such a non-contact type of infrared sensor.
A thermopile type of temperature sensor is a typical non-contact type of temperature sensor. The thermopile type of temperature sensor is provided with a thermopile element. When the thermopile element receives infrared rays, the thermopile element outputs an electrical signal having a voltage level that corresponds to the radiation intensity of the received infrared rays. In general, a heat-generating substance such as a heating roller radiates infrared rays with a radiation intensity that corresponds to the surface temperature thereof. For that reason, the thermopile type of temperature sensor can be used to detect the surface temperature of the heating roller, if it receives the infrared rays radiating from the heating roller.
However, since an infrared sensor generally has a low thermal resistance, it is difficult to dispose the infrared sensor in the vicinity of a heating roller that is maintained at a high heat fixing temperature. It is therefore necessary to dispose the infrared sensor and the heating roller with a predetermined spacing therebetween. If the spacing is too far, the infrared sensor will sense the temperature of portions other than the surface of the heating roller, which may lead to errors in the sensing of the surface temperature of the heating roller.
In Japanese unexamined patent application publication No. 7-77891, it is proposed to dispose the heating roller and the infrared sensor at a predetermined spacing apart, and interpose a cylindrical member between the heating roller and the infrared sensor in order to limit the viewing angle.
However, if a cylindrical member is interposed between the heating roller and the infrared sensor, the infrared rays radiating from the heating roller attenuates as they pass through the cylindrical member. The viewing angle of the infrared sensor is determined by the diameter of the cylindrical member and the distance from the infrared sensor to the leading end of the cylindrical member. Accordingly, if the spacing between the infrared sensor and the heating roller is made larger, the viewing angle will be narrowed by a corresponding amount. This is inconvenient in that the radiation intensity of infrared rays sensed by the infrared sensor is not so sufficient as to achieve accurate temperature sensing.
In addition, when a non-contact type of temperature sensor described above is used, the heat generated by the heating sensor will transfer the non-contact type of temperature sensor through the conduit, so that the temperature of the non-contact type of temperature sensor will gradually rise. In such a case, since the non-contact type of temperature sensor usually has a lower thermal resistance, the usage of the non-contact type of temperature sensor may raise problems in that the temperature sensing accuracy may fall and the non-contact type of temperature sensor may deteriorate.
Furthermore, if the attachment position of the non-contact type of temperature sensor with respect to the heating roller is displaced, the quantity of infrared rays arriving at the non-contact type of temperature sensor will change, raising a problem in that the temperature sensing accuracy will fall.